For me, for peace of mind and emergencies I always have a SWR amplite + DI pedal + other bits and bobs such as fuses, tape, tools etc, all in a small bag which lives in my car, beside the spare tire. Hidden under the floor. Also in the bag is a nappy; it stops things getting damp. I replace this once in a while, normally when I get my gear PAT tested.
This kit has been very useful: I've leant bits out to other muso's over the years, and I like having the piece of mind that this offers. As an example: the SWR has been used in a past emergency to power a small vocal PA at a wedding; I was a dep bassist for that gig, and the band were grateful I had this to hand when their gear failed. I've played for this band many times since and it always comes up in conversation how I saved the day.
For me, my 'mini emergency kit' is as useful as the spare tire it sits alongside; rarely used but nice to know it's there!
I always take two basses in my Mono Dual bass gig bag; usually a four and a five, but depends on the gig. It's easy to swap basses if a string snaps mid-song; been there, done that.
On upright gigs, I always carry a spare fretless electric - especially for weddings in marquees. I has amazed me over the years how low the ceiling height is where the band is asked to play; even if the contract stipulates 7ft clearance required. I am 6ft 3 and need a minimum of 7ft (ideally higher), clearance for upright bass. My fretless has been used many times in these situations.